The invention relates to an apparatus for selectively supplying hydraulic fluid discharged from a plurality of pumps to a hydraulic unit.
Considering a power steering system, for example, which is mounted on an automobile to alleviate the magnitude of a force which is required for a driver to operate a steering wheel, a pump may be used as a source of oil pressure. Such pump is driven for rotation by an engine of the automobile, and has a discharge rate which increases or decreases in proportion to the number of revolutions of the engine. Accordingly, it is necessary that such pump has a sufficient capacity to supply enough fluid to operate the hydraulic unit such as a power steering device properly even if the engine operates at a low number of revolutions or with a reduced discharge from the pump.
However, it will be seen that if the pump is provided with such capacity, it follows that an unnecessarily large amount of fluid is supplied when the engine operates at a higher number of revolutions. This not only results in a waste in itself, but also increases the dissipation of the horsepower of the engine which is used to drive the pump, which is undesirable from the viewpoint of power saving. In addition, a recent trend is directed to improving the fuel cost of the engines mounted on automobiles, and hence it is desired that the dissipation of the horsepower which is utilized to drive the pump associated with the power steering device be minimized.
To cope with this problem, a variety of apparatuses for supplying hydraulic fluid have been proposed in the prior art, including a single pump cartridge having a pair of pressure chambers which are disposed in symmetrical relationship with each other and which are separated from each other to provide a pair of pumps, and the use of a pair of pumps, each of a reduced capacity, and which are connected to a flow path switching mechanism so that hydaulic oil therefrom can be selectively supplied. Specifically, the switching mechanism is operated in a manner such that normally only one of the pumps is utilized to supply the hydraulic oil while the other is connected to an associated tank to present no load, thus attempting to achieve a reduction in the dissipation of the horsepower. However, when necessary, the hydraulic oil from the both pumps is combined for supply to a hydraulic unit. Also, several different techniques are employed to control the supply of such hydraulic oil. By way of example, in an arrangement which senses the number of revolutions, the hydraulic oil from the both pumps is combined when the number of revolutions of the engine is in its low range or in response to the flow whenever the discharge from the respective pumps is low. In another arrangement of the pressure sensing type, the hydraulic oil from both pumps is combined in response to the detection of a hydraulic pressure which is produced whenever a hydraulic unit is loaded to actuate a power steering device and irrespective of the magnitude of the number of revolutions of the engine.
In the former arrangement, when the automobile is running at a high speed, or in a high range of the number of revolutions of the engine, only one of the pumps is driven, thus permitting a reduction in the dissipated horsepower to be achieved. However, in a low range of the number of revolutions of the engine, a power loss is unavoidable, leaving much to be improved. This is attributable to the fact that an increased quantity of hydraulic oil is necessary only during a steering operation when it is loaded, and the supply of the hydraulic oil may be at a lower level when the automobile is at rest or running straightforward even though the engine is rotating in its low range. In particular, 10 mode running patterns, for example, which represent a running through city areas, are most frequently utilized with automobiles, and hence it is desirable that the dissipation of the horsepower be reduced when the automobile is running at such low speeds.
In the latter arrangement, the problem associated with a low range of the number of revolutions of the engine can be eliminated to achieve a power saving, while a power loss in the high range is unavoidable. Specifically, in the low range of the number of revolutions of the engine and when the power steering device is not actuated, the other pump can be connected to the tank to present no load, thus allowing a reduction in the dissipated horsepower. However, in the high range of the number of revolutions of the engine where the single pump is able to supply sufficient hydraulic oil, the actuation of the power steering device causes the flow path to be switched in a manner to permit the hydraulic oil from the both pumps to be combined or merged. However, this results in supplying more than necessary hydraulic oil to the power steering device, resulting in a waste and causing a malfunctioning due to an excessive flow rate. To avoid this difficulty, a flow control valve is associated with the arrangement so that a constant supply can be maintained in the high range of the number of revolutions of the engine while returning an excessive amount of the hydraulic oil to the tank otherwise. However, there arises a problem of running stability.
Specifically, an increased supply of hydraulic oil when the automobile is running at a high speed, results in a reduced reaction of the steering wheel as sensed by the driver, which produces an uneasiness in the mind of the driver and thus is undesirable in providing a good steerability. In addition, it is to be noted that the steering wheel need be seldom operated through an increased stroke when running at such high speed.
It will be appreciated that an operation of the steering wheel is required when the automobile is running at low speeds. For this reason, an arrangement of the kind described is usually provided with a drooping mechanism which is associated with the flow control valve so that the supply of the hydraulic oil can be reduced to a degree in a high range of the number of revolutions of the engine. Such a drooping mechanism is often employed on passenger automobiles with a great advantage.
The use of the drooping mechanism provides a suitable reaction to the driver when maneuvering the steering wheel, since it reduces the supply below a given value in the high range of the number of revolutions of the engine, and thus improves the steerability and is most effective when the automobile is running at high speeds. Reaction sensed by the driver when operating the steering wheel is further improved by the use of the drooping mechanism since it allows a sequential increase in the supply which is necessary to compensate for an increasing load in the hydraulic pressure circuit during an operation of the steering wheel. Additionally, the drooping mechanism is effective and useful in minimizing a power loss by reducing a pressure loss which occurs across pipings and within the power steering device, through a reduction in the supply of the hydraulic oil to the power steering device when the automobile is running at high speeds.
Therefore, it has been attempted in the prior art to provide a combination of the drooping mechanism which brings forth these advantages with the hydraulic fluid supplying apparatus mentioned above which achieves a power saving. However, a problem is presented in that the described desirable drooping effects are not available when the drooping mechanism is simply added to the arrangement of pressure sensing type mentioned above unless the number of revolutions of the engine reaches a value higher than the number of revolutions at which it will be effective when the mechanism is associated with a usual pump. Thus, a problem occurs in respect of the dynamic response. This results from the fact that a conventional drooping mechanism utilizes a flow control valve which is disposed in the hydraulic pressure supply path, which valve can be operated with a supply of hydraulic oil which can be obtained only when the number of revolutions reaches a higher value than in the usual arrangement.